Synthesis of potential antitumor and antiviral nucleosides will continue with emphasis placed on the synthesis of 4'-thio-6-thioguanine, 4'-thio-6-azauridine, 4'-thio-5-azacytosine, 4'-thiotubercidin, 4'- thiosangivamycin, and 4'-thioribovirin in sufficient quantities that they can be adequately examined for expected activities.